In the past, when moldboard plows were widely used for primary tillage, either in the fall or the spring, dry fertilizer was normally spread on the surface of the soil before plowing; and it was buried deep in the soil after plowing, due to the nature of moldboard tillage. Burying fertilizer deep in the soil is advisable. When fertilizer is applied in the fall, the fertilizer can be leached out of the soil and carried away with surface drainage if it is not buried.
When the fertilizer is applied in the spring in preparation for planting, it is also desirable to bury it deeply in the soil because otherwise, as the soil becomes dry during the summer, the fertilizer is not available to the crop during hot, dry spells.
In recent years, many farmers have modified earlier tillage practices, and there has been a noticeable shift away from moldboard plowing because, among other things, moldboard plowing tends to increase soil errosion. As a result, fertilizer applicators have been developed for depositing the fertilizer deep in the ground--that is, in the range of 4-8 in. below the surface.
A knife applicator for agricultural use is not new. Typically, an applicator of this type includes a blade or knife located in a forward position for opening a slot or furrow of a desired depth and width. Typically, a conduit of closed cross section is then mounted behind the knife for guiding dry fertilizer into the slot formed behind the knife. Finally, a metal tube or rubber hose is mounted behind the dry fertilizer conduit for communicating anhydrous ammonia into the slot behind, and, hence, on top of, the dry fertilizer. Conventionally, some means is provided to close the slot with soil so that the fertilizer, once applied, remains in the soil at the desired depth.